Topical Review

Transcription

Topical Review
8619
Journal of Physiology (1999), 514.3, pp. 617—627
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Topical Review
Intrauterine nutrition: its importance during critical periods
for cardiovascular and endocrine development
J. J. Hoet and M. A. Hanson*
Laboratoire de Biologie Cellulaire, Universite Catholique de Louvain, Place Croix du Sud 5,
1348 Louvain-la-Neuve, Belgium and *Department of Obstetrics and Gynaecology,
University College London Medical School, 86—96 Chenies Mews, London WC1E 6HX, UK
(Received 26 August 1998; accepted after revision 19 November 1998)
Experimental investigations in animals have highlighted the role of early reduced calorie and
protein nutrition on fetal cardiovascular development, and the occurrence of a transition
from a low fetal arterial blood pressure in late gestation to a high arterial blood pressure
postnatally. These observations may explain the correlation between health, including
appropriate nutrition, in pregnant women and the outcome of their pregnancies. Emphasis
has been placed on low birth weight infants who have an increased risk of developing
cardiovascular diseases, including hypertension, coronary heart disease and stroke in
adulthood. Vascular pathology in adults is not always associated with low birth weight and
animal experiments indicate that substantial changes in cardiovascular and endocrine
function can result from maternal or fetal undernutrition without impairing fetal growth.
Experimental investigation on organogenesis shows the pivotal role of adequate protein
availability as well as total caloric intake. Amino acid metabolism in the feto-maternal unit
appears to have a key influence on the development of organs involved in chronic
degenerative disease in the adult. Experimental investigation has also highlighted the role of
carbohydrate metabolism and its effect on the fetus in this respect. Either restriction of
protein intake or diabetes in pregnant rats has intergenerational effects at least on the
endocrine pancreas and the brain. Further investigation is needed to clarify the mechanisms
involved and lead to a new understanding of the importance of nutrition during pregnancy.
This will provide an important approach to the primary prevention of diabetes and chronic
degenerative diseases.
The developing mammal needs to establish a degree of
autonomy during fetal life in order to achieve independent
survival after birth. It therefore develops homeostatic
mechanisms necessary to guarantee its existence. But it
passes through critical periods when it may be influenced by
aspects of the intrauterine environment dependent on
maternal nutrition and metabolism. These have been clearly
demonstrated for implantation, organogenesis and
parturition, all of which are influenced by maternal health,
including nutritional intake. But apart from the striking
effects on survival (e.g. in relation to implantation) or overt
anatomical structure (organogenesis), recent evidence
reveals that disturbances during critical periods can also
affect homeostatic mechanisms. The effects may be subtle
during development, but can nonetheless have long-lasting
deleterious effects on health in adult life.
This review concentrates primarily on how nutritional intake
during pregnancy affects cardiovascular (especially arterial
blood pressure) and blood glucose homeostasis in the
offspring. These two areas are increasingly studied in
humans (Rich-Edwards et al. 1997; Barker, 1998) and,
whilst superficially distinct, they share some common
aetiological features. To date, most of the studies in humans
have been epidemiological so that, whilst they identify
phenomena, they do not give insight into mechanisms.
Awareness of this has recently shifted emphasis to
prospective studies of smaller groups of people and to animal
studies, the latter providing the more direct approach to
understanding the processes involved. We concentrate on
these animal studies in this brief review. In some of them an
isocaloric low protein diet has been used to explore the
mechanisms by which protein metabolism affects developing
organs. Other studies have used varying degrees of global
reduction in nutrition. It is noteworthy that in some of these
studies effects on homeostatic development were produced
even in the absence of body growth restriction. It is
therefore possible to envisage a spectrum of health problems
in adult life deriving from the influence of the intrauterine
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J. J. Hoet and M. A. Hanson
environment and maternalÏplacentalÏfetal compensatory
responses to a diet mildly to severely altered in its
composition or its volume. It is also noteworthy that even if
an isocaloric low protein diet does not affect birth weight
significantly in the first generation it may reduce it in
subsequent generations. These observations are discussed
further below.
Cardiovascular development
It is well established in a variety of species that altered
nutrition can have permanent effects if it occurs at a
sensitive period during development (e.g. Winick & Noble,
1966; Osofsky, 1975; Stephens, 1980; Lechner et al. 1986;
Snoeck et al. 1990; Lucas, 1991). The effects of a period of
early undernutrition on cardiovascular function may not be
manifest until a much later stage in life. Attempts to
understand the underlying mechanisms have been made in
a variety of animals, from rodents to sheep and non-human
primates.
One of the major issues which emerges from recent work is
that birth weight does not have to be significantly reduced
for the physiological effects of reduced nutrition in
pregnancy to be manifest. This raises the possibility that
the placental or fetal compensatory mechanisms, which
occur in response to reduced maternal nutrition, preserve
normal fetal growth and hence birth weight, but have
postnatal consequences which become important in later
life. Thus the adoption of a biological ‘strategy’ needed for
development in utero results in an organism in which the
strategy for wellbeing in adulthood is not achieved.
In sheep global undernutrition in early pregnancy produces
a reduction of the birth weightÏplacental weight ratio (De
Barro et al. 1992) and in early to mid-gestation produces
increased placental size (Faichney & White, 1987; McCrabb
et al. 1991), whereas global undernutrition in late gestation
only reduces fetal growth (Mellor & Murray, 1981). Early
gestation undernutrition alters the distribution of
placentome types in favour of those with a predominant
fetal vascular pattern (Crowe et al. 1997). This pattern is
reminiscent of the increased fetal vascularization reported
in human, guinea-pig and sheep placentae at high altitude
(Bacon et al. 1984; Scheffen et al. 1990; Burton et al. 1996;
Krebs et al. 1997; Penninga & Longo, 1998) and some forms
of intrauterine growth retardation (IUGR) (see Kingdom &
Kaufmann, 1997), possibly occurring as a placental
compensatory response.
Early undernutrition affects the development and responses
of the hypothalamic—pituitary—adrenal (HPA) axis in late
gestation. Both the pituitary response to an arginine
vasopressinÏcorticotrophin releasing hormone (AVPÏCRH)
challenge and the adrenal cortical response to an
adrenocorticotrophic hormone (ACTH) challenge are
reduced (Hawkins et al. 1997, 1998). These fetuses have a
lower basal plasma cortisol concentration, which may
account for their lower arterial blood pressure (Fig. 1), as
J. Physiol. 514.3
cortisol and other glucocorticoids are known to elevate
arterial blood pressure (Tangalakis et al. 1992). Whether the
suppression of basal cortisol concentration and of HPA axis
responses is due to prior exposure of the fetus to cortisol is
not known. A hypothesis has been proposed to relate the
suppression of the HPA axis function to prior exposure of
the fetus to elevated corticosteroids (Benediktsson et al.
1993; Edwards et al. 1993) and it is already known that
dexamethasone administration suppresses HPA axis function
in the sheep (Norman & Challis, 1987).
Cardiovascular and HPA axis development are also affected
postnatally. Lambs born after periconceptual undernutrition
have higher arterial blood pressure and an exaggerated
arterial blood pressure response to an HPA axis challenge
(Hawkins et al. 1998). Their ACTH and cortisol responses
are now also greater. The mechanism for this switching from
blunted responses in utero to enhanced responses
postnatally is not yet known.
The effects of undernutrition are manifest not only in terms
of changes in reflex and endocrine control of the
cardiovascular system, but also at the level of the local
vasculature. Ozaki et al. (1998) found that the responses of
small resistance arteries, especially to endothelium
dependent vasodilator agonists, are altered in ewes and
their fetuses subjected to an early gestation nutritional
challenge. The causal role of such changes in arterial blood
pressure development remains to be determined. Similar
effects have also been reported in experimental diabetes and
with a high fat diet in the rat (Gerber et al. 1997; Koukkou
et al. 1997).
Besides undernutrition in the ewe, the removal of placental
caruncles produces fetuses with lower mean arterial blood
pressure, higher fetal heart rate and altered responses to an
episode of acute hypoxia in late gestation (Robinson et al.
1983). Fetal heart rate and the rise in arterial blood pressure
in acute hypoxia are greater in carunclectomized fetuses,
suggesting greater chemoreflex or endocrine responses even
though such fetuses are not necessarily smaller than
controls.
These carunclectomized sheep fetuses, being hypoxaemic
and hypoglycaemic have higher plasma levels of adrenaline
and noradrenaline, suggesting greater sympathetic activity.
Their higher levels of cortisol without increased ACTH
and suppressed pituitary pro-opiomelanocortin (POMC)
expression (Phillips et al. 1996) suggest that HPA axis
feedback may have been altered. In another context,
alteration of the HPA axis affects maturation of organ
systems, and may even initiate parturition (see Carmichael
et al. 1997 for discussion).
Studies in rats have shown that undernutrition in utero can
lead to life-long elevation of blood pressure. For example, an
isocaloric low protein diet (8%) during pregnancy reduces
the fetal:placental weight ratio (Levy & Jackson, 1993),
produces elevated arterial blood pressure in the offspring
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J. Physiol. 514.3
Intrauterine nutrition and development
(Langley & Jackson, 1994), via changes in angiotensin
converting enzyme activity (Langley-Evans & Jackson,
1995), and produces changes in glutathione metabolism and
glucose tolerance as well as insulin secretion (Langley &
Jackson, 1994; Langley-Evans & Jackson, 1995). In
subsequent experiments it was shown that induction of
hypertension by reduced maternal protein was abolished if
cortisol synthesis was inhibited. This led to the hypothesis
that maternal protein restriction programmes life-long
changes in the fetal HPA axis, which in turn resets
homeostatic mechanisms controlling blood pressure.
Maternal protein restriction attenuates activity of placental
11â-HSD (hydroxysteroid dehydrogenase) type II and an
alternative explanation is that fetal blood pressure is altered
through increased exposure to maternal glucocorticoids
(Edwards et al. 1993). In this species, placental 11â-HSD
activity is correlated with birth weight and inversely
correlated with placental weight (Bendiktsson et al. 1993),
and inhibition of 11â-HSD with carbenoxolone in
pregnancy produces offspring with a higher arterial blood
pressure (Langley-Evans, 1997). A third possibility is that
placental activity of the enzyme plays a crucial role in the
development of the fetal adrenal and hence may determine
patterns of glucocorticoid secretion throughout life. There
may also be effects on the kidney (see below) and on
vascular growth and metabolism.
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The effects of severe total dietary restriction have also been
examined in the rat. Woodall et al. (1996) reduced diet by
75% and reported elevated blood pressure. However,
Holemans et al. (1998) found that severe restriction in late
gestation did not produce hypertensive offspring. It appears
that the timing of the insult in gestation is all-important,
with insults occurring earlier having greater effects on
cardiovascular development in the offspring.
The experiments on the rat appear to differ from those in
the sheep in that cardiovascular and endocrine changes are
associated with growth retardation. Methodological
differences and the role of undernutrition versus isocaloric
protein restriction have, however, to be noted. In the rat the
level of reduction in maternal carbohydrate andÏor protein
intake has usually been based on requirements at the time
of conception, and maintained throughout pregnancy.
Because dietary requirement increased throughout
pregnancy, this may constitute a challenge of increasing
severity. Recently, in a series of studies in which intake was
reduced by 30% of the appropriate daily requirement
throughout gestation, only male pups were smaller than
control, at least at the first generation. Nonetheless, they
showed perturbed vascular development, particularly in
arterial blood pressure and in responses of small vessels to
vasodilator agonists (Ozaki et al. 1998) (Fig. 2). In addition,
Figure 1. Basal mean arterial pressure (A) and ACTH response to administration of
CRH + AVP (B) in control (5) and nutrient restricted (4) fetuses (113—127 days gestation) and
lambs (84 ± 4·4 days)
Values are means ± s.e.m. Arterial pressure is shown as the mean of measurements made over a 2 week
period in the fetus, and as the mean of measurements made on a single day in the lamb. ACTH responses to
CRH + AVP challenge were measured for 180 min after drug administration in the fetus, and for 60 min
in the lamb; ACTH data are shown as the cumulative response over the first 15 min following drug
administration, with samples taken every 5 min. Blood pressure data were compared by Student’s
unpaired t test, and ACTH data by two-way analysis of variance followed by Dunnett’s post-hoc test
(*P < 0·05). Fetal basal blood pressure, and ACTH responses to CRH + AVP were significantly reduced
following maternal nutrient restriction. Postnatally, basal blood pressure and ACTH responses were
significantly greater in lambs of undernourished mothers. Adapted from Hawkins et al. (1997, 1998).
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J. J. Hoet and M. A. Hanson
a recent report on the growth of the spontaneously hypertensive rat (SHR; Lewis et al. 1997; Fig. 3) shows that the
pups of this strain are not smaller than normotensive pups
at birth, although as fetuses they were smaller at days
16—20 of gestation. Their greater late gestation growth than
controls is associated with a significantly larger placenta.
Thus whatever the mechanisms for the earlier impaired
growth, fetal growth in late gestation was accelerated by a
greater placental mass. However, the latter has consequences
for subsequent postnatal cardiovascular function.
Spontaneously hypertensive rat pups had larger hearts and
kidneys, which may be associated with the cardiovascular
effects. Somewhat similar observations have been made in
the rat using iron deficiency anaemia. This reduces birth
weight and neonatal mean arterial blood pressure, but the
pups become hypertensive relative to controls when
postnatal ‘catch-up’ growth occurs and their heart size
increases (Crowe et al. 1995).
J. Physiol. 514.3
Blood pressures of SHR are permanently lowered if they are
suckled by dams of normotensive strains for 2 weeks after
birth. Milk of SHR differs from milk of other strains in
protein and electrolyte concentrations (Di Nicolantonio,
1993; McCarty & Lee, 1994; McCarty & Fields-Okotcha,
1996). Other studies have shown that the intrauterine
environment of the SHR is different, as the amniotic fluid
has high osmolarity and sodium concentrations (Erkadius
et al. 1994). Altering the intrauterine environment
experimentally by inducing maternal diabetes also affects
blood pressure in the SHR.
Finally, in the guinea-pig maternal undernutrition impairs
placental development and fetal growth (Dwyer et al. 1992)
and it has also been shown that unilateral ligation of the
uterine artery during pregnancy indeed leads to reduced
birth weight in the offspring and raised blood pressure after
birth. The mechanism underlying this phenomenon is
unknown but increased circulating catecholamines have
been proposed (Persson & Jansson, 1992).
Figure 2. Effects of a 30% restriction in global diet during pregnancy on the mean arterial
pressure (A) and responses of small resistance arteries to the thromboxane Aµ mimetic U46619
in vitro (B) of male offspring
Offspring of nutritionally restriced dams (NR) show higher arterial blood pressure at both 100 and 200 days
postnatally, compared with controls (C). In addition, small artery contractile responses, measured as
maximal tension (Emax) to U46619 as a percentage of maximal K¤-induced tension, are greater in NR than
in C at both ages. All data are means ± s.e.m. *P < 0·05 NR vs. C by the Mann—Whitney U test. Data
from Ozaki et al. (1998).
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J. Physiol. 514.3
Intrauterine nutrition and development
Endocrine pancreas
Besides the vasculature, the development of other key
tissues may be affected in the offspring of ewes or dams
having global nutritional restriction. However a specific
limitation of protein in a normocaloric diet which prevents
an energy constraint has been explored widely in pregnant
animals and found to affect the offspring’s islet cells (Snoeck
et al. 1990), insulin sensitive tissues such as liver (Hales et al.
1996), muscle (Ozanne et al. 1996a,b) and adipose tissue
(Shepherd et al. 1997) as well as the kidney (MerletBenichou et al. 1994) and brain (Resnick et al. 1979). With
this experimental dietary approach particular mechanisms
operating in the fetomaternal unit which are dependent
upon specific nutritional intake of the mother might be
explored. The later part of this review deals with the
consequences for the offspring of dams receiving a moderate
protein restriction in a normocaloric diet.
In vitro , fetal â cell differentiation, multiplication and
insulin secretion are amplified more by an increase in
essential amino acid than by an increase in glucose
concentration (de Gasparo et al. 1978). This suggests a
specific role for appropriate protein availability upon the
development of fetal â cells, acting via changes in amino
acid metabolism. This raises the possibility of abnormal
features being acquired by the developing â cell which may
lead to pathological events postnatally, as a result of a
protein deficient diet of the dam.
With an isocaloric low protein (8%) diet during gestation
the profile of amino acids is changed in maternal and fetal
plasma as well as in amniotic fluid. The total essential and
non-essential amino acid concentrations were not modified,
nor were glucose and insulin levels (Reusens et al. 1995).
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More specifically, á_aminobutyric acid, phosphoserine,
taurine and valine were reduced in maternal as well as in
fetal plasma. The weight of the fetus was normal at
19·5 days but slightly lower at birth compared with the
controls.
The endocrine pancreas was abnormal in that islet cell
proliferation (not evenly distributed in the head and tail of
the endocrine pancreas), islet cell size, pancreatic insulin
content and islet vascular density were all reduced at birth
(Snoeck et al. 1990). Furthermore the ontogeny of the
endocrine pancreas of offspring from these dams was
perturbed when the low protein diet was maintained during
the suckling period. At each fetal and postnatal day
analysed, the number of apoptotic cells in these islets was
increased while the number of cells positive for insulin-like
growth factor II (IGF-II), considered to be a survival factor
which prevents apoptosis (Petrik et al. 1998), was decreased
(Reusens et al. 1998). The function of these fetal â cells was
affected with insulin secretion being diminished by 50% in
vitro compared with controls (Dahri et al. 1994). With a
normal diet postnatally, insulin secretion in vitro remained
impaired when stimulated by arginine and leucine (Dahri et
al. 1994). In vivo the adult offspring showed normal basal
plasma glucose and insulin levels as well as a normal amino
acid profile. During a glucose challenge, the insulin level was
abnormally low in adult non-pregnant females and it
remained low during pregnancy, associated with higher
than normal plasma glucose levels (Table 1).
When the isocaloric low protein diet was maintained
postnatally the adult offspring showed an abnormal amino
acid profile which was associated with a reduced volume of
the endocrine pancreas and pancreatic insulin content
Figure 3. Weight (means ± s.d.) of the fetus (A) and placenta (B) in late gestation in
spontaneously hypertensive rats (SHR; 4) and normotensive Wistar—Kyoto controls (WKY; 5)
The lower fetal weight in mid-gestation in SHR is not seen in late gestation, and this effect is associated
with greater placental weight. Note that gestation is slightly longer in SHR than WKY. *P less than 0·01
by unpaired t test; **P less than 0·01 by ANOVA, SHR vs. WKY. Adapted from Lewis et al. (1997).
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J. J. Hoet and M. A. Hanson
J. Physiol. 514.3
––––––––––––––––––––––––––––––––––––––––––––––
Table 1. Effects of protein deficiency during pregnancy on the endocrine pancreas of
the adult offspring
––––––––––––––––––––––––––––––––––––––––––––––
Low diet maintained postnatally
Normal diet given postnatally
––––––––––––––––––––––––––––––––––––––––––––––
Islet cell mass d
Pancreatic insulin content d
â cell sensitivity to glucose and amino acids in vitro d â cell sensitivity to amino acids in vitro d
Plasma insulin level d
Insulin response to oral glucose challenge d
Insulin response to oral glucose only in female d
––––––––––––––––––––––––––––––––––––––––––––––
(Dahri et al. 1995). Islet blood vessel density (Dahri et al.
1993) as well as pancreatic and islet blood flow (IglesiasBarreira et al. 1996) were also diminished. It was of
particular interest that reduced activity of mitochondrial
glycerophosphate dehydrogenase (mGPDH) was observed in
these islets (Rasschaert et al. 1995). A similar reduction is
observed in islet cells of human subjects with type 2 diabetes.
Special emphasis should be placed on the sensitivity of fetal
islets to taurine, which is an indispensable amino acid
during fetal and neonatal development in rats, cats and
baboons (Sturman, 1993). Plasma taurine was significantly
reduced in dams fed low protein in gestation, and in their
fetuses. Insulin secretion by normal fetal islets was
stimulated in vitro by taurine and, when added to the
culture medium, it enhanced insulin secretion in response to
other secretagogues (Cherif et al. 1996). Islets of fetuses
from dams on a low protein diet did not secrete insulin in
response to taurine and in the culture medium it did not
restore a normal secretory response to other secretagogues.
However, when taurine was added to the drinking water of
rats on a low protein isocaloric diet, to re-establish plasma
taurine levels in dams and fetuses, the insulin secretion in
response to taurine and other secretagogues was restored to
normal (Cherif et al. 1998). Thus, current observations point
to the need for normal protein availability in the diet and
identify taurine as a necessary amino acid for the normal
functional development of fetal â cells.
A low protein isocaloric diet also has an intergenerational
effect on birth weight and the endocrine pancreas (Hoet et
al. 1997). This diet reduced the birth weight of pups and it
reduced it further in subsequent generations, producing an
increased number of growth restricted pups (Stewart, 1975).
As noted above, the female offspring showed abnormally
low plasma insulin and high plasma glucose levels in
response to an oral glucose challenge when they were
pregnant. Their pups had lower plasma insulin and lower
insulin content and reduced volume density of the
endocrine pancreas (Table 2). When the low protein intake
was maintained postnatally as well as during gestation, the
mother and the pups showed higher plasma glucose and
lower plasma insulin levels than normal. Alterations of
endocrine pancreas and its insulin content in these pups
were even more marked than in the previous generation.
Therefore chronic isocaloric protein deprivation, which
lowered the feto-maternal levels of amino acids such as
taurine, initiates intergenerational effects on glucose
homeostasis and the development of the endocrine pancreas.
A similar intergenerational effect of a protein restricted diet
has already been observed on brain development and
tryptophan metabolism (Resnick & Morgan, 1984).
Insulin sensitive tissues
Many aspects of liver function, including cholesterol
synthesis and fibrinogen production, are differentially
expressed in the periportal and perivenous zones. The livers
of pups born to mothers on an isocaloric low protein diet
underwent changes in zonation and enzyme activity,
including a reduction in glucokinase and an increase in
phosphoenolpyruvate carboxykinase activity. These were not
restored at adulthood even when the animals were nourished
with a normal diet (Desai et al. 1995) and were responsible for
changes in the regulation of hepatic glucose output (Ozanne et
al. 1996a). Altered zonation is likely to be linked to other
important changes in hepatic function, though these are as
yet unknown. There are, however, indications that
manipulations during gestation up-regulated cholesterol
synthesis (Innis, 1983, 1985). The number of insulin
receptors was increased in the liver, skeletal muscle (Ozanne
et al. 1996a,b) and white fat adipocytes (Shepherd et al.
1997). In addition the adipocytes were smaller and did not
show changes in GLUT_4 expression, although this was
increased in the plasma membrane of skeletal muscle
(Ozanne et al. 1996b). The adipocytes of adults had a
greater glucose uptake and a higher phosphatidylinositol
3_kinase activity (Ozanne et al. 1997). Adipose tissue of
offspring was also affected by global dietary restriction,
which comprises low protein availability in the dams. In this
instance white adipose tissue increased and brown adipose
tissue decreased in the adult, possibly indicating lower
sympathetic activity. Rats whose mothers had restricted
food during the first 2 weeks of pregnancy indeed became
obese, but depending on the strain and the diet used it was
either the males or the females which were affected (Jones &
Friedman, 1982; Anguita 1993).
Thus limited protein intake during gestation leads to
alterations in glucose output by the liver as well as in the
sensitivity of tissues to insulin. Glucose transporters in
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Intrauterine nutrition and development
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––––––––––––––––––––––––––––––––––––––––––––––
Table 2. Effects of protein deficiency during pregnancy on insulin glucose homeostasis
in 1st and 2nd generation offspring
––––––––––––––––––––––––––––––––––––––––––––––
First generation
Second generation
pregnant female offspring
late gestation fetus
–––––––––––––––
––––––––––––––
Low protein
Normal
Low protein
Normal
postnatal diet
postnatal diet
postnatal diet postnatal diet
––––––––––––––––––––––––––––––––––––––––––––––
Plasma glucose
+
n.e.
+
n.e.
Plasma insulin
−
n.e.
−
−
Pancreatic insulin content
−
−
−
−
Insulin response to OGTT
−
––
––
––
Islet cell mass
––
––
−
−
––––––––––––––––––––––––––––––––––––––––––––––
+, increase; −, decrease; n.e., no effect; ––, not measured.
––––––––––––––––––––––––––––––––––––––––––––––
muscles and the expression of key components of insulin
signalling pathways in adipocytes are also altered. In
addition, studies have suggested that maternal dietary
restriction during gestation and lactation and transient
dietary protein restriction after weaning may permanently
alter growth hormone secretion in offspring (Harel &
Tannenbaum, 1995).
Kidney
In non-human primates and rats neonatal kidney weights
were reduced by an isocaloric low protein diet given to the
mother. Hence there appears to be a specific effect of protein
limitation on the kidney (Cheek & Hill, 1975; MerletBenichou et al. 1994). In rats a severe low protein isocaloric
diet during pregnancy is associated with low birth weight
and a significant reduction in the number of mature glomeruli
(Merlet-Benichou et al. 1994) with an increase in the number
of immature glomeruli (Zeman, 1968) in the progeny. Kidney
weight as well as the final number of mature glomeruli
remained reduced at 14 days postnatally even when the pups
were fed a normal diet postnatally. Therefore an isocaloric
low protein diet during pregnancy leads to permanent
changes in the kidney which are not reversible postnatally.
The number of renal glomeruli is also reported to be reduced
in infants who are malnourished in early life (Naeye, 1965;
Hinchliffe et al. 1992) as well as in adults with hypertension
(Hayman et al. 1939, Mackenzie et Brenner, 1995). Lastly,
the number of nephrons in humans is known to be
correlated with birth weight (Merlet-Benichou et al. 1993).
Brain
In the brain of offspring of dams exposed experimentally to
an isocaloric low protein (8%) diet before and during
pregnancy, there were anatomical and physiological
alterations (Resnick et al. 1979). The distribution and levels
of biogenic amines were changed and there were
modifications of tryptophan metabolism. Whilst a normal
diet postnatally restores some of these features, the changes
in biogenic amines persist (Resnick et al. 1979). These
alterations became even more severe when the same low
protein isocaloric diet was given to subsequent generations
(Resnick & Morgan, 1984). Thus for brain development a low
protein isocaloric diet initiates an inter-generational effect
similar to that shown for the endocrine pancreas. In
addition, brain blood vessel density was reduced in pups
born to dams on an isocaloric low protein diet (Reusens et al.
1997) and remained reduced when a normal diet was given
postnatally. This contrasts with the restoration of blood
vessel density in the endocrine pancreas, which was
restored by giving a normal diet postnatally (Dahri et al.
1993). The mechanisms involved in the effects of nutrition
on vasculogenesis in various tissues are not yet known.
Comparison of dietary restriction with maternal diabetes
Besides maternal nutritional deprivation, with protein
reduction or global dietary reduction, maternal diabetes is
also known to affect fetal tissue development in humans and
experimental animals. Streptozotocin-induced diabetes in a
mild form induced a higher birth weight than normal, an
increased percentage of pancreatic endocrine tissue with
â cell hyperplasia and an increase in the number of
degranulated cells (Aerts et al. 1990). Hence the proliferative
capacity of the islet was increased in vivo, and it remained
elevated when cultured in a normal medium for 7 days
(Reusens et al. 1984). In the gut, villous and microvillous
surface area was increased in the duodenum, jejunum and
ileum, and mucosal blood vessel density was also increased
(Reusens et al. 1989). However when dams had severe
diabetes, the fetal weight, islet size and â cell mass were
decreased (Aerts et al. 1990) and atrophy of the fetal
intestinal tract occurred (Reusens et al. 1989). In such
maternal diabetes, induced experimentally by streptozotocin,
the pups of a second generation showed a reduction in birth
weight together with permanent changes in endocrine
pancreatic structure and function (Aerts et al. 1990). Pups
from moderately as well as severely diabetic dams became
diabetic at adulthood. Pancreatic insulin depletion in the
former and insulin resistance in the latter were causal in
initiating the diabetes in the offspring postnatally (Holemans
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J. J. Hoet and M. A. Hanson
et al. 1991). There are in addition effects of maternal
streptozotocin-induced diabetes on the responses of small
resistance arteries in the pups (see Koukkou et al. 1997).
Both the experimental conditions of maternal protein
restriction and diabetes stress the impact of maternal
nutritional limitation or poor health in producing effects on
the offspring. Several tissues are affected, including the
vasculature, endocrine pancreas, insulin sensitive tissues,
kidney and brain. Some of the pathological changes can
develop after a delay and there are inter-generational effects.
The effects may cause degenerative diseases in adults and
they can occur without major changes in birth weight.
Therefore birth weight may only be a poor proxy for intrauterine events.
Worldwide perspective
In considering sources of variation in fetal growth, the focus
in this review has been on nutrition, with particular
reference to the nutritional programming hypothesis. There
are, however, many factors affecting fetal growth and size at
birth: the fetal genotype, the maternal genotype, the
mother’s pre-pregnancy nutritional status, her metabolism
and physiology, her diet during pregnancy, and the
resultant hormonal and circulatory milieu which sustains
fetal growth. At least as far as the protein restriction model
is concerned, we have stressed the importance of the
quantity and quality of amino acids available to the fetus.
But for all the factors involved, there is a need to determine
the specific mechanisms responsible for the alterations in
fetal development.
Birth weight is a crude measure of fetal growth: babies of
the same weight may, for example, be short and fat or long
and thin, and may be markedly different in organ size and
structure, physiology and metabolism. In humans, low birth
weight is the result of multifactorial processes operating
during pregnancy. It is confounded by the influence of
ethnic origin, low socio-economic status and poor nutrition
which may affect the mother and her offspring
concomitantly. Data from developing countries indicate that
in some areas of Asia, 20% of women have stunted growth
with an adult height of below 1·45 m, and 65% have an
adult body weight below 45 kg. More than 35% of mothers
deliver infants with birth weights under 2·5 kg (Galloway &
Anderson, 1994). Much of the situation stems from a povertyrelated deficiency of protein and energy intake as well as of
key micronutrients such as vitamin A, iodine and iron, from
which women and children suffer disproportionately.
Inadequate diets are also associated with infectious diseases
which, when occurring during pregnancy, increase the
energy, protein and micronutrient needs and are associated
with low birth weight. In infants up to 5 years of age
proteinÏenergy malnutrition contributes 12·7% of the total
burden of diseases. Malnutrition is also sex linked and severe
malnutrition affects seven times more female than male
infants in the developing world (World Development Report,
1993). Vitamin A, iodine or iron deficiency during pregnancy
J. Physiol. 514.3
and early life each have a specific and immense impact on
the offspring’s health, contributing 11·7%, 7·2% and 14%,
respectively, to the total disease burden throughout the
developing world. Recently an epidemic of diabetes and
cardiovascular disease in the younger age group has become
apparent in these countries. In western countries,
epidemiological surveys highlight the importance of low
birth weight in increasing the risk of developing diabetes
and cardiovascular disease in adulthood (Hales, 1996;
Rich_Edwards et al. 1997). A study in 15000 Swedish men
and women provides by far the most convincing evidence of
a true association between size at birth and mortality from
ischaemic heart disease, and it strongly suggests that it is
variation in fetal growth rate rather than size at birth that is
important for aetiology (Leon et al. 1998). Low birth weight
(< 2·5 kg) and poor early growth (< 8 kg at age 1 year) are
associated with a high incidence (more than 45%) of
diabetes, impaired glucose tolerance andÏor myocardial
infarction at 65 years of age. However, in twin pregnancies,
the baby with the lower birth weight is more likely to
develop type 2 diabetes than its twin, indicating that the
intrauterine environment of the individual fetus is more
important in the aetiology of this disease than genetics.
Thinness at birth is also associated with insulin resistance
later (Phillips et al. 1994).
Epidemiological studies of degenerative diseases should now
focus on nutritional conditions across generations, to relate
events occurring early in life to the time span for
pathological conditions to appear. Recently the first studies
on this topic have been published. Examination of maternal
diet during pregnancy and health of the offspring suggests
that in humans nutrition during pregnancy does indeed
have long-term consequences for the health of the offspring.
For example, in Aberdeen the blood pressures of men and
women were found to be related to the balance of animal
protein and carbohydrate in their mother’s diet during late
pregnancy (Campbell, 1996). This association did not
depend on birth weight. The protein intake of the mother is
also related to the offspring’s glucose—insulin metabolism
(Barker, 1998). Low protein intake was found to be
associated with insulin resistance, though the association
was weaker than that with low maternal body mass.
Recently, it has been shown that people exposed to the
Dutch famine of 1944—1945 in utero had higher plasma
glucose and insulin concentrations after a standard glucose
load, suggesting that their poor glucose tolerance was
mainly determined by insulin resistance (Ravelli et al.
1998). The effects of famine were independent of size at
birth. These findings provide direct evidence that
undernutrition in utero is a key factor in the aetiology of
non-insulin dependent diabetes mellitus, and show that the
mother’s dietary intake during pregnancy can program
metabolism without altering size at birth.
It is thus clear from a range of human and animal studies
that poor maternal health andÏor nutritional deficiencies
affect key tissues during their development, and can be
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J. Physiol. 514.3
Intrauterine nutrition and development
responsible for pathological changes in the offspring. Specific
protein malnutrition during human pregnancy and children’s
health has not been explored extensively although
experimental observations suggest new approaches to
establishing a correlation. In the human, neonatal hypercysteinemia and hypercysteinuria are associated with
vascular damage in early life (McCully, 1969). Increased
plasma levels of homocysteine have been found to be
associated with increased risk of myocardial infarction in
adulthood possibly due to enzymatic alterations associated
with vitamin B6 or vitamin B12 activity. One may surmise
that perturbed amino acid homeostasis during pregnancy
may initiate fetal developmental abnormalities leading to
pathological events in later life.
We suggest that extensive animal studies are now needed to
discover specific mechanisms by which altered fetomaternal
nutrition and amino acid metabolism lead to degenerative
diseases in the offspring. Only when the causes in early life
of these human diseases are established, will it be possible
to devise ways for their primary prevention.
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We would like to thank Professor C. Remacle and Dr B. Reusens for
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Reusens, B., Hill, D. J., Petrik, J., Remacle, C. & Hoet, J. J.
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M. Hanson: Department of Obstetrics and Gynaecology, University
College London Medical School, 86—96 Chenies Mews, London
WC1E 6HX, UK.
Email: [email protected]
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Corresponding author
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